Known in the art are acoustic surface wave transducers of the interdigital type, comprising a first contact area and a second contact area disposed on a piezoelectric substrate and a row of electrodes arranged on the piezoelectric substrate and consisting of a first group of electrodes galvanically coupled to the first contact area and a second group of electrodes in parallel to the electrodes of the first group and arranged therebetween so as to form overlapping portions of the electrodes of both groups, the second group of electrodes being galvanically coupled to the second contact area.
It is known (cf. R. H. Tancrell, M. G. Holland "Acoustic Surface Wave Filters", Par. IEEE, vol. 59, No. 3, pp. 393-409, Mar. 1977) that the amplitude-frequency response of an acoustic surface wave transducer is a Fourier transform of its impulse response. When designing such a transducer, the first step comprises, as a rule, calculation of its impulse response, which provides an amplitude-frequency response curve closest in shape to the specified response. In order to obtain a specified impulse response, the intensity of excitation of the acoustic surface waves by each pair of adjacent electrodes must be weighted.
In the above described transducer, this weighing is effected by changing the length of the overlapping portions of the adjacent electrodes of the first and second groups in accordance with a given law of amplitude modulation in the impulse response. Such transducers are known as apodized transducers.
In the known filters, in which the input and output acoustic surface wave transducers are located in a single acoustic channel, one of the transducers, e.g. the input one, is made as an apodized transducer.
An apodized transducer excites acoustic surface waves with a nonuniform wave front along the beam aperture, because each pair of overlapping adjacent electrodes excites acoustic surface waves with a beam width corresponding to the length of the overlapping area. Therefore, the output transducer of the filter must have the same length of the overlapping sections to provide for a transform of the law of change of the electrode overlapping of the input transducer into amplitude modulation of the impulse response without distortion. Thus, in the known filter, the amplitude-frequency response is provided in one transducer only, which does not make it possible to obtain a high level of signal suppression outside of the limits of the filter pass band. Furthermore, with a small area of the overlapping portions of the adjacent electrodes of the apodized transducer, diffraction effects become manifest so that the front of the acoustic surface waves excited by these electrodes is converted from planar into a circular one. This distorts the impulse response of the filter and increases energy losses in its pass band.
In order to increase the level of suppression of the signal beyond the filter pass band, it is desirable to shape its amplitude-frequency response both in the input and output transducers. It can be made in a filter, in which both transducers are apodized and arranged in parallel acoustic channels interconnected through a multistrip system of electrodes (cf. J. M. Deacon, J. Housway, J. A. Jenkins "Multistrip Coupler in Acoustic Surface Wave Filters", Electr. Let., v. 9, No. 10, p. 235, 1973).
The amplitude-frequency response of such a filter is a product of the amplitude-frequency responses in the input and output transducers, which provides for better suppression of the signal outside of the filter pass band. However, the diffraction effects and associated disadvantages are not eliminated in this filter. Additional losses of energy in the pass band of such a filter occur during the transfer of the acoustic surface waves from the acoustic channel of the input transducer into the acoustic channel of the output transducer.
Also known in the art are acoustic surface wave transducers, in which the weighing of the intensity of excitation of the acoustic surface waves is effected by changing the capacitances coupling the transducer electrodes with the contact areas (cf. U.S. Pat. No. 3,904,996; cl. 333-72).
In one embodiment of such a transducer, each contact area has a surface layer of a dielectric, on which there are arranged sections of electrodes of a corresponding group so as to provide a capacitor between each electrode and the corresponding contact area. The capacitance of this capacitor is defined by the area of said portions of the electrodes and is varied by changing the area of these portions from electrode to electrode in accordance with the specified impulse response of the transducer. Such a weighing technique makes it possible to obtain transducer electrodes having the same overlapping length along the entire length of the transducer so that all pairs of adjacent electrodes excite acoustic surface waves with an identical width of the beam but with a different amplitude. When such a transducer is used in a filter, the second transducer can be apodized, which allows the amplitude-frequency response of the filter to be shaped in both transducers.
However, the presence of a dielectric layer on the contact areas results in a number of significant disadvantages of the above described design of the transducer. First of all, the process of manufacture of the transducer is considerably complicated since it includes several steps; making of contact areas, application of a dielectric layer thereon while providing precision checking of its thickness, and making of electrodes. When the electrodes are being made using a photolithographic technique, the photostencil has to be placed on the dielectric layer whose thickness is commensurate with the width of the electrodes. As a result, a gap is formed between the photostencil and the substrate. This causes erosion of the electrode edges and limits the highest possible operating frequency of the transducer compared to the transducers described above. Furthermore, punctures and other defects of the dielectric layer are possible, which result in bridging the electrodes with the contact areas and, therefore, in reduction of the output of suitable products.
It has been proposed to use acoustic surface wave interdigital transducers comprising a first and second contact areas arranged on a piezoelectric substrate and a main row of electrodes located on the piezoelectric substrate and comprising a first group of electrodes electrically connected to the first contact area and a second group of electrodes parallel to the electrodes of the first group and arranged therebetween so as to form overlapping portions of electrodes of both groups having the same length and coupled to the second contact area through capacitors whose capacitances are defined by a specified impulse response of the transducer (cf. U.S. Pat. No. 3,904,996, Cl. 333-72).
In such a transducer, the electrical coupling of the electrodes of the first group to the first contact area is also provided through capacitors whose capacitances are defined by a specified impulse response of the transducer. These capacitors are formed by the recesses of the contact areas and the ends of the electrodes entering these recesses. The capacitance of each capacitor depends on the depth of the recess and the width of the gap between the end of the corresponding electrode and the contact area. A required law of change of the capacitances of the capacitors along the length of the transducer is provided by changing the depth of the recesses and the width of the gap between the ends of the corresponding electrodes and the contact area, i.e. of two parameters. This considerably hampers the calculation of the transducer.
Such a design of the transducer results in high energy losses and distortion of the amplitude-frequency response within the pass band of this device and also in a low level of suppression of the signal outside of this band. All this stems from the fact that the acoustic surface waves within the pass band of the transducer are excited not only at the points of overlapping of the electrodes of the first and second groups but also in the gaps between the contact areas and the electrodes of the corresponding groups. In this case, the above mentioned diffraction effects appear at portions with shallow recesses. These effects result in distortion of the impulse response and, therefore, in distortion of the amplitude-frequency response in the pass band of the transducer and poor suppression of the signal outside of the pass band.
Acoustic surface wave filters were proposed comprising an input and output interdigital transducers, in which the main rows of electrodes are arranged on a piezoelectric substrate in a single acoustic channel (cf. U.S. Pat. No. 3,904,996; cl. 333-72).
In such a filter, one transducer, e.g. the input one, is made as described above, while the output transducer is apodized. On the other hand, the output transducer may be made identical to the input one.
This filter has all disadvantages inherent in the above described acoustic surface wave transducer.